The present invention relates generally to methods and apparatus for generating multiple frequency tones, and more particularly, to such methods and apparatus that employ high speed digital frequency dividing to generate multiple frequency tones and provide for a small, low power, low cost multiple frequency tone generator.
Typical multiple, harmonically-related tone generators, also called "comb generators," use one of two methods. One method is to generate harmonically related tones using an "impulse function" methods. This method takes a single frequency tone from a reference source and processes it through a circuit that generates an impulse signal, or very short duration pulse from the signal. This short pulse has sharp leading and trailing edges and a short pulse duration results in the generation of multiple harmonics of the reference frequency tone comprising the generating signal. The quantity and level of the harmonics can be controlled by varying the pulse duration and the rise and fall time of the impulse signal. The hardware needed to generate this short duration pulse can be complex and difficult, especially if multiple harmonics are required. The resulting signal must be divided into multiple paths, and each path individually buffered and filtered. Because the impulse method can require additional signals (such as the control signal that gates the short duration pulse), filtering of the output signal can be difficult.
The primary disadvantages of the impulse method of comb generation is the more complex nature of the hardware needed to generate the short duration pulse needed to generate multiple harmonically related signals. This method has been widely used, and will continue to be used for applications where the frequency or bandwidth of the desired signals are outside the capabilities of available digital dividers.
A second method of generating multiple frequency tones is to use multiple, separate reference signal sources. A device employing this method is very large and costly since a separate signal source is needed for each desired signal tone. A slight variation on this approach is to use other components such as mixers, frequency multipliers or dividers, or other similar hardware to generate some of the tones. The disadvantage of these variations of the basic method is that each tone generated needs separate hardware. Once again each output signal must be individually buffered and filtered to assure interference free output tones.
The primary disadvantage of the multiple source method is the quantity and cost of the hardware needed to generate the desired tones. Its reference source generator typically comprises a large physical portion of the device, and since each output signal requires a separate reference source, the net result is a very large (and quite costly) implementation. The advantage of the multiple source method is that the outputs do not have to be harmonically related.
In view of the above, and in order to provide for an improvement over the above-cited conventional techniques, it is an objective of the present invention to provide for means and methods for generating multiple frequency tones that employs high speed digital frequency divider devices to generate multiple frequency tones.